The present invention relates to the field of transporting very viscous fluids such as extra heavy crude oils, bitumen or tar sands which hereinafter will be refered to as viscous oils.
Friction losses are often encountered during the pumping of viscous fluids through a pipeline. These losses are due to the shear stresses between the pipe wall and the fluid being transported. When these friction losses are great, significant pressure drops occur along the pipeline. In extreme situations, the viscous fluid being transported can stick to the pipe walls, particularly at sites which are sharp changes in the flow direction.
A known procedure for reducing friction losses within the pipeline is the introduction of a less viscous immiscible fluid such as water into the flow to act as a lubricating layer for absorbing the shear stress existing between the walls of the pipe and the fluid. This procedure is known as core flow because of the formation of a stable core of the more viscous fluid, i.e. the viscous oil, and a surrounding, generally annular, layer of less viscous fluid. U.S. Pat. Nos. 2,821,205 to Chilton et al. and 3,977,469 to Broussard et al. illustrate the use of core flow during the pipeline transmission of oil.
Normally, core flow is established by injecting the less viscous fluid around the more viscous fluid being pumped in the pipeline. U.S. Pat. No. 3,502,103 and 3,826,279, both to Verschuur, and U.S. Pat. No. 3,886,972 to Scott et al. illustrate some of the devices used to create core flow within a pipeline. An alternative approach for establishing core flow is illustrated in U.S. Pat. No. 4,047,539 to Kruka wherein the core flow is created by subjecting a water-in-oil emulsion to a high shear rate.
Although fresh water is the most common fluid used as the less viscous component of the core flow, other fluids or a combination of water with additives have been used. U.S. Pat. No. 3,892,252 to Poettman illustrates a method for increasing the flow capacity of a pipeline used to transport fluids by introducing a micellar system into the fluid flow. The micellar system comprises a surfactant, water and a hydrocarbon. U.S.S.R. Pat. No. 485,277 to Avdshiev illustrates a method where the lower viscosity fluid is formed by an emulsion of a light fraction of hydrocarbon in water. U.S.S.R. Pat. No. 767,451 to Budina et al. illustrates a core flow method wherein the lower viscosity fluid is a solution of water and synthetic tensoactive agents.
In any normal crude oil pumping operation, there exists a significant possibility of a breakdown which interrupts the operation. For example, the mechanical failure of a pump, an electrical power failure or a break in the pipeline can interrupt the flow of oil through the pipeline. When core flow is being used to transport viscous oil through a pipeline, interruptions in operation for relatively short time periods can cause stratification to occur between the phases. Attempts to restrart the core flow by simultaneously starting the low viscosity fluid and viscous oil pumps can create large pressure peaks at the discharge of the pumps or along the pipeline. These large pressure peaks can cause the failure of the pipeline because the pressure could exceed the allowable maximum working pressure.
Accordingly, it is an object of the present invention to provide a process for restarting core flow within a pipeline.
It is a further object of the present invention to provide a process as above which substantially reduces the maximum pressure encountered during start-up.
It is yet a further object of the present invention to provide a process as above which substantially eliminates large pressure fluctuations in the system.
These and other objects and advantages will become more apparent from the following description and drawings in which like reference numerals depict like elements.